Posted By
JamesD
on 2013-04-20
15:18:20
|  Re: Flash memory through 3+1 ROM sockets?
It is certainly possible to replace the ROMs with Flash memory.
You could fit every ROM in the Plus/4 and then some on a single FLASH chip.
There are some potential issues though.
The most obvious problem is you have to have a circuit to convert from one pinout to the other. This isn't difficult but you either have to hand wire a board or use a printed circuit board. The board will also need to accommodate any new circuitry.
You need to convert from separate chip selects to a single chip select with additional address lines. This isn't that difficult, if any chip select is active then you activate the chip select on the FLASH (active low signal I believe) and you pass the existing chip selects through something like a 74LS148 (or 147) to output the new address lines on the FLASH chip. You just need to have the inputs in the proper order for the content of the FLASH chip to make some sense when you create the original content for the FLASH chip.
And last but certainly not least, you can't write to the FLASH memory while it's in the machine without running a write signal from the CPU to the FLASH chip. And that assumes the existing address decoding doesn't ignore writes. If it doesn't activate the chip selects during a write to those addresses, you'd have to create completely new address decoding and I suggest you skip the idea of writing to the FLASH memory while it's in the machine.
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